Audio quality, such as voice audio quality, is an important aspect of communication systems. Audio quality expectations among users is grow at an accelerated rate. Many users experience high levels of voice quality when using high bandwidth communication systems, such as Voice of Internet Protocol (VoIP). High Definition (HD) Voice was created to provide improved voice quality for mobile communications. HD Voice provides improved voice quality by utilizing wider audio bandwidth up to 8 kHz provided by the Adaptive Multi-Rate-WideBand (AMR-WB) codec. This improvement in audio quality improves the clarity of speech and provides a more natural feel to mobile conversations. HD Voice doubles the audio bandwidth compared to traditional telephony.
The 3rd Generation Partnership Project (3GPP) has developed an Enhanced Voice Services (EVS) codec that provides high quality voice and general audio, such as music, and provides low latency communication well suited for real-time communication. While the EVS codec was developed with 3rd Generation Partnership Project Long Term Evolution (LTE) in mind, the EVS codec is also useful for Voice over IP (VoIP) and Circuit-Switched (CS) systems. Relative to previous voice codecs, the EVS codec provides improved audio quality for same bitrate. This improved audio quality at the same bitrate provides improved network capacity while maintaining audio quality. The EVS codec extends the audio bandwidth up to 20 kHz covering the full range of human hearing. The EVS codec also provides high quality audio not just for voice communications, but for generic audio such as music. The EVS codec was developed specifically for Internet Protocol (IP) based communications with improved resilience to packet loss and delay jitter. Therefore, the EVS codec not only covers the full range of human hearing, but also addresses the pitfalls of IP communications.
The EVS codec improves perceived audio quality and improves coding efficiency for narrowband and wideband audio bandwidths using a wide range of bitrates starting from 7.2 kbits per second. Non-fixed coding rates are also available in the EVS codec. A source controlled variable bitrate (SC-VBR) mode at an average bitrate of 5.9 kbits per second is supported for narrowband and wideband audio. The EVS codec provides an average bitrate starting from 9.6 kbits per second for super wideband (SWB) and starting from 16.4 kbits per second for fullband (FB). The maximum bitrate for the EVS codec is 24.4 kbits per second for narrowband and 128 kbits per second for all other audio bandwidths.
The EVS codec provides input and output sampling at 8, 16, 32 and 48 kHz. To optimize perceptual coding quality, an integrated bandwidth detector automatically adapts the actual bandwidth of the input signal, that may be a lower bandwidth than the bandwidth indicated to the EVS codec. This automatic adaptation of the bitrate allows the EVS codec to easily adapt to changes in channel capacity.
The EVS codec utilizes discontinuous transmission (DTX) including algorithms for voice activity detection (VAD) and comfort noise generation (CNG). In the SC-VBR coding mode, the DTX/CNG algorithms are always used for interactive speech coding. An advanced error concealment mechanism mitigates the quality impact of channel errors resulting in lost packets. The EVS codec also includes a system for jitter buffer management (JBM) to address the jitter. Jitter is the variation in the delay of the received packets. The EVS codec also includes a channel-aware mode to increase robustness in the presence of adverse channel conditions. The channel-aware mode operates at 13.2 kbits per second for both wideband and super wideband audio. The EVS codec also provides backward compatibility with the Adaptive Multi-Rate Wideband (AMR-WB) codec through an interoperable (IO) mode.
While the above mentioned improvement provided by the EVS codec provide greatly improved audio quality, improvements are needed regarding the controlling the adoption of the EVS codec and controlling the bitrate of the EVS codec once adopted.